最近在用kotlin编写从assets复制文件到CacheDir的代码，想在StackOverflow上找一下最佳实践，发现有的人在调用input.copyTo(output, 1024)之后会调用output.flush()而有的人不会，于是研究了一下这个问题：FileOutputStream是否需要flush和close?
答案：需要close()但不需要flush()。因为在FileOutputStream中，override了close方法进行了一些操作比如关闭channel等等，没有overrideflush方法；而在它的父类OutputStream的代码中，close和flush的实现均为空。

OutputStream.java

  /**
    * Flushes this output stream and forces any buffered output bytes
    * to be written out. The general contract of <code>flush</code> is
    * that calling it is an indication that, if any bytes previously
    * written have been buffered by the implementation of the output
    * stream, such bytes should immediately be written to their
    * intended destination.

    * If the intended destination of this stream is an abstraction provided by
    * the underlying operating system, for example a file, then flushing the
    * stream guarantees only that bytes previously written to the stream are
    * passed to the operating system for writing; it does not guarantee that
    * they are actually written to a physical device such as a disk drive.

    * The <code>flush</code> method of <code>OutputStream</code> does nothing.
    *
    * @exception  IOException  if an I/O error occurs.
    */
    public void flush() throws IOException {
    }

   /**
    * Closes this output stream and releases any system resources
    * associated with this stream. The general contract of <code>close</code>
    * is that it closes the output stream. A closed stream cannot perform
    * output operations and cannot be reopened.

    * The <code>close</code> method of <code>OutputStream</code> does nothing.     

    * @exception  IOException  if an I/O error occurs.
    */
    public void close() throws IOException {
    }

FileOutputStream.java

public void close() throws IOException {
    synchronized (closeLock) {
        if (closed) {
            return;
        }
        closed = true;
    }

    // Android-added: CloseGuard support.
    guard.close();
    if (channel != null) {
        channel.close();
    }

    // BEGIN Android-changed: Close handling / notification of blocked threads.
    if (isFdOwner) {
        IoBridge.closeAndSignalBlockedThreads(fd);
    }
    // END Android-changed: Close handling / notification of blocked threads.
}